High pressure, flexible hoses for conveying fluids under high pressure such as hydraulic oil, water, but also steam or gasses are omnipresent in all kinds of equipment and vehicles to effectively transfer motion or fluids between parts that can move relative to one another.
Typically these kind of hoses consist of an elastomer core tube that contains the fluid around which one or more reinforcement layers have been spirally wound while being radially separated by intermediate layers of elastomer material. An outer layer is generally applied on the outermost reinforcement layer for covering the reinforcement such that it does not get damaged.
The reinforcement layer is crucial to the functioning of the hose in that it does not only impart strength to the hose but also keeps the hose flexible. The reinforcement layer must be able to withstand highly variable pressures with shock waves propagating through the fluid. As hoses are many times used in harsh environments and convey aggressive fluids at high or low temperatures the reinforcement layer must maintain its properties of strength, flexibility and fatigue resistance also in these circumstances.
While in the past expectations from novel synthetic reinforcement fibers such as long chain synthetic polyamide based aramids, or ultra-high molecular weight polyethylene oriented fibres were high, the most used material for reinforcing high pressure hose is still the plain steel wire, possibly coated with an adhesive layer.
The steel wires can be applied around the core tube in a number of ways such as in:                Spiralled hoses wherein the wires are wound parallel to one another in a helicoidal way. Preferably the reinforcement layers are paired and wires of members of a pair are wound in opposite directions while being separated by an intermediate polymer or elastomer layer. Or        The reinforcement layer can be braided which can be described as a pair of spiral layers wound closely together without an intermediate elastomer layer. The wires of each spiral layers are grouped in ribbons, and the ribbons of one layer are interwoven with the ribbons of the other layer. The interweaving can be according a plain weave, twill weave or satin weave.        
The steel wires in the hose are wound under a helix angle that is conventionally set as the angle between the tangent vector to the steel wire and the axis of the helix. When the angle is close to the so called ‘neutral angle’ the reinforcement will behave neutral when pressure is applied to the hose: the hose will neither shorten nor elongate. The neutral angle is A tan(√{square root over (2)}) which is about 54°44′.
From the mechanical point of view the hose must fulfil some basic requirements:                It must have sufficient burst pressure (BP) i.e. the pressure at which the weakest wall part of a test piece yields. The burst pressure is amongst other factors directly depending on the breaking load and the number of reinforcing fibres in the reinforcement layer. Other factors are the construct of the hose: number of layers, radial positions of layers, lay angles, spiralled or braided, rubbers used, etc.        A hose must be used below its burst pressure at a certain working pressure (WP). Depending on the application of the hose a safety factor is used that for hydraulic applications is generally set to 4. The pressure at which the hose is intended to be used should then not be larger than one quarter of the burst pressure.        As hoses are subject to impulse pressures for example due to pumping cycle, valve closing, fluid hammer and other abuse impulse life testing is used to simulate the effect of those phenomena. During impulse life testing a piece of hose is square wave pressure loaded from near zero up to 125% or 133% (depending on hose construction) of the working pressure and back with a certain repetition rate and duty cycle. The test is either stopped due to failure of the hose or when a prescribed number of cycles has been reached. Possibly a surviving test piece is subject to a final burst test to assess the final performance of the hose.        
The following prior art in the field of steel wire reinforced high pressure hoses is of particular relevance to the invention:                JP 2005 291466 (Yokohama Rubber Co Ltd) describes a hose that comprises layers of steel filaments that have received a two-dimensional waveform or a three dimensional spiral shape before incorporation of the wires into the reinforcement layers of the hose. The waveform or spiral shape must be such that the wire elongates at least 0.35 at 20% of its breaking load, but not more than 15%. The purpose is to dampen pulsation waves through the hose. The wires are of equal make throughout the hose. The drawback is that not all wires may be loaded to the maximum of their capabilities.        U.S. Pat. No. 4,273,160 (Parker-Hannifin) describes a flexible high pressure hose with at least two layers of stranded reinforcement, wherein the modulus of elasticity of the material within one layer increases when radially going outward. The text mentions the use of metal wires only for the outer layer, while the inner layers are made of synthetic fibres. Hence two different types of material are needed.        WO 2007/020503 (Eaton Corporation) describes a high pressure hose comprising at least two reinforcement layers with different tensile strength of which the inner layer has a lower tensile strength than the outer layer. The assumption is that a lower tensile strength of the wire leads to an increased fatigue life of the steel wire and hence an increased pulse life. Different tensile classes of steel wire are used within a single hose. No mention is made of the elongation at break of those wires.        EP 0200253 (Bekaert) describes a method to condition a hose reinforced with steel wires that all have been stress relieved in order to have a higher elongation at break. By applying a high ‘preconditioning pressure’ of more than 90% of the burst pressure of the hose, the wires are permanently elongated at the optimal load sharing. The method is an additional extra treatment that is not without risk.        
The prior art hoses therefore suffer from the mentioned drawbacks that the inventors solved with the below described innovative high pressure hose.